In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. This is particularly true for multicolor cinematic projection systems. In order to provide a competitive alternative to conventional cinematic-quality projectors, digital projection apparatus must meet high standards of performance, providing high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 1,000:1.
The most promising solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators. The first type of spatial light modulator is a Digital Micromirror Device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat. No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951 (Anderson); and U.S. Pat. No. 6,089,717 (Iwai). DMDs have been employed in digital projection systems. However, although DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, inherent resolution limitations (with current devices providing only 1024×768 pixels) and high component and system costs have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is a Liquid Crystal Device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size and favorable device yields. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,221 (Maki et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.). Exemplary LCD projection patents U.S. Pat. No. 4,425,028 (Gagnon); U.S. Pat. No. 4,749,259 (Ledebuhr); and U.S. Pat. No. 4,911,547 (Ledebuhr) describe dual color polarization designs that provide improved color performance.
In an electronic projection apparatus using spatial light modulators, individual colors, conventionally red, green, and blue (RGB), are separately modulated in a corresponding red, green, or blue portion of the optical path. The modulated light of each color is then combined in order to form a composite, multicolor RGB color image. There are two basic approaches for projection optics that combine the modulated color light. The first approach, which can be characterized as a convergent approach, is adapted from earlier, conventional projection systems. Using the convergent approach, the component red, green, and blue light have separate axes which are converged by projection optics that effectively bend each light path as necessary in order to form a composite, multicolor color image at some focal plane. As an illustrative example, U.S. Pat. No. 5,345,262 (Yee et al.) discloses a convergent video projection system. Significantly, the disclosure of U.S. Pat. No. 5,345,262 illustrates one of the major problems with the convergent projection approach: namely, that the separate color images must be properly registered on the projection surface. Misregistration or poor focus along any one of the color light projection paths can easily result in an unsatisfactory image. It is instructive to observe that, using this approach, the image paths are converged only at the focus plane.
U.S. Pat. No. 5,907,437 (Sprotbery et al.) discloses an attempt to simplify design complexity and alleviate some of the light path alignment and registration problems inherent to multicolor projection systems using the convergent approach described above. In the U.S. Pat. No. 5,907,437 disclosure, a light valve projection system is described in which a converging optical system converges the red, green, and blue modulated light paths in order to form a converged image, advantageously centered on the axis of a projection lens. The design strategy outlined in U.S. Pat. No. 5,907,437 thus simplifies the projection lens design task for a system using the convergent approach. However, other problems inherent to a convergent approach remain.
One notable problem with approaches similar to that disclosed in U.S. Pat. No. 5,907,437 is a relatively high etendue. As is well known in the optical arts, etendue relates to the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image. Numerically, etendue is proportional to the product of two factors, namely the image area and the square of the numerical aperture (NA). Increasing the numerical aperture, for example, increases etendue so that the optical system captures more light. Similarly, increasing the source image size, so that light originates over a larger area, increases etendue and, therefore, brightness. As a general rule, increased etendue results in a more complex and costly optical design. Using an approach such as that outlined in U.S. Pat. No. 5,907,437, for example, lens components in the optical system must be designed for large etendue. The source image area for the light that must be converged through system optics is the sum of the combined areas of the spatial light modulators in red, green, and blue light paths; notably, this is three times the area of the final multicolor image formed. That is, for the configuration disclosed in U.S. Pat. No. 5,907,437, optical components handle a sizable image area, therefore a high etendue, since red, green, and blue color paths are separate and must be optically converged. Moreover, although the configuration disclosed in U.S. Pat. No. 5,907,437 handles light from three times the area of the final multicolor image formed, this configuration does not afford any benefit of increased brightness, since each color path contains only one-third of the total light level. In particular, the second relay lens and the projection lens of a convergent optics system such as that disclosed in U.S. Pat. No. 5,907,437 are inherently constrained by a large etendue, which adds cost and complexity to such a solution. Moreover, the second relay lens must be color corrected over the full visible spectrum. At the same time, different segments of the relay lens and of the projection lens handle different wavelengths, so that localized lens imperfections, dust, or dirt not only affect the projected image, but can impact the color quality. In light, then, of etendue constraints, of color correction requirements, of dust and dirt sensitivity, and of the need for maximizing brightness levels for digital projection, there appear to be significant inherent limitations that hamper the convergent approach exemplified in U.S. Pat. No. 5,907,437.
An alternative approach to projection optics can be characterized as a coaxial approach. In contrast to the convergent approach in which component red, green and blue light beams are bent to converge at a focal plane, the coaxial approach combines the component red, green, and blue modulated light beams along a common axis. In order to do this, the coaxial approach employs a dichroic combining element, such as an X-cube or Philips prism. X-cubes or X-prisms and related dichroic optical elements, such as those disclosed in U.S. Pat. No. 5,098,183 (Sonehara) and U.S. Pat. No. 6,019,474 (Doany et al.) are well known in the optical imaging arts. The dichroic combining element combines modulated light from each color path and folds the color paths together along a common axis in order to provide the combined color image to a projection lens. Referring to FIG. 1, there is shown a simplified block diagram of a conventional digital projection apparatus 10 using the coaxial approach. Each color path (r=Red, g=Green, b=Blue) uses similar components for forming a modulated light beam. Individual components within each path are labeled with an appended r, g, or b, appropriately. For the description that follows, however, distinctions between color paths are specified only when necessary. Following any of the three color paths, a light source 20 provides unmodulated light, which is conditioned by uniformizing optics 22 to provide a uniform illumination. A polarizing beamsplitter 24 directs light having the appropriate polarization state to a spatial light modulator 30 which selectively modulates the polarization state of the incident light over an array of pixel sites. The action of spatial light modulator 30 forms an image. The modulated light from this image, transmitted along an optical axis Or, Og, Ob through polarizing beamsplitter 24, is directed to a dichroic combiner 26, typically an X-cube, Philips prism, or combination of dichroic surfaces in conventional systems. Dichroic combiner 26 combines the red, green, and blue modulated images from separate optical axes Or, Og, Ob to form a combined, multicolor image for a projection lens 32 along a common optical axis O for projection onto a display surface 40, such as a projection screen.
In contrast to the convergent approach outlined above with reference to U.S. Pat. No. 5,907,437, the coaxial approach, as shown in the block diagram of FIG. 1 and as exemplified in U.S. Pat. No. 5,808,795 has a number of advantages. With respect to light throughput, the coaxial approach, because it combines light paths along a common axis, does not increase the etendue of the optical system. Instead, with respect to projection lens 32, dichroic combiner 26, by folding the appropriate optical axes Or and Ob to join with optical axis Og and form a common optical axis O, optically overlaps the areas of spatial light modulators 30r, 30g, 30b. Thus, the etendue has no increase whether one, two, three, or more spatial light modulators are combined in this way. Since each light color is separately modulated, then combined and provided to projection lens 32 along a common optical axis O, no optical system is required between dichroic combiner 26 and projection lens 32.
A Philips prism, such as that disclosed in U.S. Pat. No. 3,202,039 (DeLang et al.) could alternately be employed as dichroic combiner 26. Familiar to those skilled in the digital image projection arts, Philips prisms have been employed as chromatic separator or combiner components in projector designs such as those disclosed in U.S. Pat. Nos. 6,280,035 and 6,172,813 (both to Tadic-Galeb et al.); U.S. Pat. No. 6,262,851 (Marshall); and U.S. Pat. No. 5,621,486 (Doany et al.), for example.
While digital projection apparatus 10 designed using the basic model of FIG. 1 are able to provide good levels of image quality, there is felt to be room for improvement. Constraints imposed by dichroic coatings are a key consideration. Dichroic coatings used for dichroic combiner 26 can be expensive and difficult to design and fabricate for suitable performance with incident light over a wide range of angles, particularly in projection applications where high brightness levels and a broad color gamut are needed. Dichroic coatings reflect and transmit light as a function of incident angle and wavelength. As the incident angle varies, the wavelength of light that is transmitted or reflected also changes. Where a dichroic coating is used with an optical system having a low f/#, a broad spectrum will be reflected or transmitted by the coating, due to the wide range of incident angles.
FIGS. 2a and 2b illustrate the change in the performance of a dichroic coating as the range of incident light angles increases. Referring to FIGS. 2a and 2b, there is represented a light cone from a point source P and incident to a dichroic surface 36, which is disposed at a diagonal in these Figures. FIGS. 2a and 2b represent light incident to a dichroic surface 36 at two different f/# values. In FIG. 2a, the light cone, having a smaller f/#, is incident to dichroic surface 36 at a larger range of angles. Incident angles are considered with respect to a normal N to dichroic surface 36. Because of the difference between angle A at one extreme of the incident light beam and angle B at the opposite extreme, dichroic surface 36 will cause a color shift to occur across the transmitted and reflected light cones.
By comparison, the light cone is incident at a larger f/# in FIG. 2b. Here, there is very little difference between angles A′ and B′ at extreme edges of the incident light cone. In such a case, dichroic surface 36 response will cause correspondingly less color shift across the transmitted and reflected light cones than with the smaller f/# shown in FIG. 2a. 
As is clear from FIGS. 2a and 2b, dichroic surface 36 has some support structure, typically a prism 42. For minimizing aberrations, the flatness of surfaces 44a and 44b is of more importance with the smaller f/# of FIG. 2a where the light cone presents a wider angular range, than with the larger f/# of FIG. 2b where the light cone presents a smaller angular range. Thus, if a larger f/# light cone can be used, surface tolerance requirements of prism 42 in a dichroic combiner can be relaxed, thereby reducing cost and alignment complexity. However, conventionally, a light cone having a smaller f/# is used in projection systems, since system designs are directed to maximizing brightness.
Related to the benefits of higher f/# illustrated in FIGS. 2a and 2b are the benefits of telecentricity illustrated by comparing FIGS. 2c and 2d. Here, point sources P1, P2, and P3 represent points on a flat image plane, with rays incident on dichroic surface 36. In FIG. 2c, light cones from point sources P1, P2, and P3 are telecentric and corresponding angles C and D are identical. By comparison, in FIG. 2d, the light cones are not telecentric and corresponding angles C′ and D′ differ. This difference in incident angles can cause light from point source P1 to have a slightly different color than light from point source P3, thereby producing a color shift over the field.
It may be observed that FIGS. 2a-2d, as drawn, do not show the refraction of light at the glass-air interface. As is well known, some refraction would occur, modifying the angles of incident light to some degree.
From FIGS. 2a-2d, it can be seen that there are advantages in providing telecentric light at a small range of incidence angles (that is, at high f/#). However, in practice, it has been shown to be difficult to obtain these advantages for digital projection apparatus, due to the need to maintain high brightness levels at the same time.
As is well known in the imaging arts, in order to maximize color gamut, each composite color should have a narrow spectral range, to provide as pure a saturated color as possible. For example, it would be difficult to produce a deep red color using a red light channel that also includes some green light. Thus, where a low f/# is used with a color-combining prism or other optical element, the broad spectral response of the dichroic coating reduces the color gamut. At the same time, however, a low f/#, because it collects more light at a wider angular range, is desirable for obtaining high brightness levels. While there are conventional corrective techniques for improving color gamut, such as filtering, these techniques can reduce brightness. The limitations of coatings within dichroic combiner 26, then, constrain the capability of projection apparatus 10 optics to optimize both brightness level and color gamut.
FIG. 3a illustrates the response of an idealized X-cube 126 to light directed through a lens 132 from a light source P. X-cube 126 comprises four prisms 128a, 128b, 128c, and 128d, having appropriate surface treatments and cemented together. Surfaces 130a and 130b are treated to reflect light from P having the proper wavelength. Surfaces 130a and 130b must be perfectly planar and perfectly aligned with respect to one another to provide aberration-free reflection in order to form an image at point P′. In contrast with FIG. 3a, FIG. 3b shows the response of X-cube 126′ in which surfaces 130a and 130b are not perfectly aligned. The misalignment of surfaces 130a and 130b causes light from point source P to form two separate images at points P1′ and P2′. Aberration effects, such as the visible seam noted above, would be most noticeable when the distance between image points P1′ and P2′ is on the order of one pixel width or larger. Clearly, blur in an image that is reflected through X-cube 126′ would be increasingly more pronounced with increasing distance, in any direction, between image points P1′ and P2′. Correspondingly, the smaller the pixel dimensions, the greater the impact of an aberration due to imperfections in fabrication of X-cube 126′.
Limitations of the familiar X-cube dichroic combiner 26 are inherent in the manufacture of the device itself. The X-cube is assembled from four prisms, with the internal surfaces of each prism serving as substrates which support the appropriate dichroic coatings. In fabrication, prisms are glued together, with inner surface planes aligned as closely as possible. However, even slight tolerance errors in X-cube fabrication can result in imaging problems when these devices are used with conventional digital projection solutions. For example, slight misalignment of the planar coated surfaces within the X-cube could cause aberrations such as color fringing. Color fringing aberrations can be corrected up to a point, however, there would be advantages to a dichroic combiner design that was less prone to problems of this type. The “seam” at which dichroic surfaces are combined tends to appear as one or more linear shadow artifacts in the displayed image. Fabrication of a high-quality X-cube is further complicated by the requirement that individual component prisms have identical refractive indices; in practice, this is best accomplished when the same glass melt is used for all prism components. In addition, coating processes must be uniform on multiple prism surfaces. Optical coatings applied to X-cube surfaces typically have sub-micron thickness and are multi-layered, sometimes requiring as many as 50 or 60 layers. Thus, careful parts tracking must be carried out in addition to precision fabrication practices. Further problems are a result of the difficulties in providing uniform, flat surfaces on outer faces of the assembled X-cube. It can be well appreciated that this complexity adds considerable cost to the X-cube. Finally, obtaining brightness using conventional approaches results in high heat levels, which can damage adhesives and coating surfaces of the X-cube.
It can be readily appreciated that, in practice, fabrication of a perfect X-cube 126 as in FIG. 3a would be very difficult and some tolerance for error must be allowed. For this reason, it is advantageous, when designing an optical system using an X-cube 126, to minimize dependence on maintaining precise X-cube 126 tolerances.
Of related importance for imaging quality is preserving telecentricity wherever possible in the optical system. As was described above, particularly with reference to FIGS. 2a-2d, it is beneficial to minimize angular differences for light incident at any dichroic surface in the optical system, both for color-separating and for color-combining dichroics. In addition, it is also well known in the optical design arts that telecentric imaging at spatial light modulator 30 helps to further reduce contrast shading across the image. When cones of light from two different point source locations on the surface of spatial light modulator 30 are not telecentric, these different locations then present different incident angles to dichroic surfaces in dichroic combiner 26. In response, dichroic combiner 26 reflects different wavelength bands at different field positions, resulting in color shifts across the image.
As another well known principle in design of projection apparatus, it is beneficial to minimize the retrofocus distance of projection lens 32, thus minimizing the back working distance requirements and cost of projection lens 32. It would be preferable to avoid the cost and complexity requirements of a projection lens having a long back focal length relative to its effective focal length, such as the solution disclosed in U.S. Pat. No. 6,008,951 (Anderson), for example.
U.S. Pat. No. 6,113,239 (Sampsell et al.) discloses an arrangement of projection display components with a hybrid X-cube design that incrementally reduces the back working distance requirement for a projection lens. In this disclosure, both polarizing beamsplitters and dichroic combining surfaces are combined in a single X-cube, shortening the projection lens working distance when compared with other prior art designs. Notably, however, improved brightness is not achieved by the design in U.S. Pat. No. 6,113,239, since the angular constraints of dichroic surfaces have not been alleviated. Other problems include costly coatings solutions, since polarizing beamsplitter coatings are not readily optimized for all color and polarization combinations. Moreover, further improvement in the working distance requirements would be beneficial.
U.S. Pat. No. 5,944,401 (Murakami et al.) discloses, as an alternative to X-cube dichroics, an optical block comprising dichroic surfaces within plastic prisms. This solution provides some relief for back working distance requirements, since the refractive index of plastics exceeds that of air. To minimize back working distance, transmissive spatial light modulators are employed, allowing image-formation as close to the combining optical block as possible. However, this arrangement would not be well-suited for projector apparatus using reflective spatial light modulators, since back working distance requirements are still excessive. In terms of back working distance, the solution of U.S. Pat. No. 5,944,401 is not advantaged over conventional X-cube designs. A sizable projection lens would be required for full-scale cinema projection.
Moreover, the solution disclosed in U.S. Pat. No. 5,944,401 does not address the inherent angular limitations of dichroic surfaces described above. Thus, brightness levels are constrained with this type of design solution.
U.S. Pat. No. 5,597,222 (Doany et al.) discloses, for use in a digital projector, an optical relay lens system that alleviates some of the difficulties noted above that relate to inherent tolerance problems and projection lens working requirements. U.S. Pat. No. 5,597,222 discloses the use of a single 1×, double-telecentric relay lens to relay the combined image from individual RGB color paths to a MacNeille polarizing beamsplitter (PBS), also termed a polarization beamsplitter. In U.S. Pat. No. 5,597,222 spatial light modulators are disposed very near a dichroic combiner X-cube, to minimize thereby some of the potential adverse effects of imperfections in outer surface flatness and tolerance errors in inner surface fabrication. The system disclosed in U.S. Pat. No. 5,597,222 is advantaged in that the design of its projection lens is simplified when compared with similar designs. The working distance requirements for the projection lens are significantly reduced using the design approach of U.S. Pat. No. 5,597,222. The single 1× double telecentric relay provides the necessary working distance to allow insertion of the MacNeille PBS prior to the intermediate internal combined image in the image path. The projection lens can then re-image this internal image to the screen without the requirements for long working distance that are typically required when using a PBS and/or a dichroic color combiner, such as an X-prism. However, the solution presented in U.S. Pat. No. 5,597,222 falls far short of what is needed to compensate for inherent problems with X-cube coatings and surfaces so that both image brightness and color gamut can be maintained. For example, the design noted in U.S. Pat. No. 5,597,222 fails to address inherent angular dependencies in the dichroic coating response, so that it remains difficult to support a large color gamut while maintaining image brightness at the same time. Moreover, the projection lens must also use a high numerical aperture with this design, which implies added cost over designs with lower numerical aperture. Because of the scale of spatial light modulator components, the design of U.S. Pat. No. 5,597,222 is still very dependent on high-quality X-cube design. Further, the arrangement disclosed in U.S. Pat. No. 5,597,222 employs a relatively large number of optical components between a polarizing beamsplitter and its modulating LCD. With a large number of optical components in the path of a polarized illumination source, some unavoidable stress birefringence would necessarily alter the polarization states of both unmodulated and modulated light traveling in both directions, resulting in loss of image contrast.
U.S. Pat. No. 5,357,289 (Konno et al.) discloses a system that, similar to that disclosed U.S. Pat. No. 5,597,222 uses a single 1× relay lens to present an internal intermediate image to the projection lens, thereby significantly reducing the working distance requirements imposed on projection lens design. U.S. Pat. No. 5,357,289 provides an alternate construction to that shown in U.S. Pat. No. 5,597,222 for using polarization and color combining prisms. In the apparatus of U.S. Pat. No. 5,357,289, both the polarizing and color-combining prism are in the vicinity of the spatial light modulators, rather than spaced well apart, as in the apparatus of U.S. Pat. No. 5,597,222. Instead of the conventional X-prism, the apparatus of U.S. Pat. No. 5,357,289 uses a V-prism as a color combiner, where the V-prism is similar to that disclosed in U.S. Pat. No. 5,944,401 described above. The V-prism approach avoids some of the inherent problems with X-cube fabrication and use. While the approach disclosed in U.S. Pat. No. 5,357,289 eases the demands on projection lens design, the imaging relay (first lens group) presents a challenge, since it must provide a long working distance for the spatial light modulators and associated PBS and color-combining V-prism. As with the approach noted in U.S. Pat. No. 5,597,222, the approach shown in U.S. Pat. No. 5,357,289 uses a single imaging relay lens for all three colors (RGB), operating nominally at 1× magnification. As was seen with the U.S. Pat. No. 5,597,222 apparatus, the U.S. Pat. No. 5,357,289 approach requires a complex imaging relay lens that is fully color corrected over a broad part of the visible spectrum in order to form a white light image having minimal color aberrations or color differences in the third order aberrations, particularly with respect to distortion and defocus.
U.S. Pat. No. 6,247,816 (Cipolla et al.) discloses use of a 1×relay lens for relaying an intermediate image towards a dichroic combiner in only one of the color paths. The solution in U.S. Pat. No. 6,247,816 addresses a component packaging problem, but does not alleviate any of the angular constraints imposed by dichroic combiner response. Neither does the solution in U.S. Pat. No. 6,247,816 provide any relief with respect to back working distance requirements of the projection lens.
U.S. Pat. No. 4,836,649 (Ledebuhr et al.) discloses a 1× relay lens arrangement used in the illumination path to minimize the size of polarization components and in the modulated light path to help alleviate back working distance constraints for a digital projection system. While this arrangement provides some advantages, color-combining dichroic surfaces must still handle light at low f/# values, resulting in reduced color gamut. Moreover, the projection lens must also operate at a low f/# when using this solution.
As is stressed in the disclosure of U.S. Pat. No. 5,374,968 (Haven et al.) the conventional approach for maximizing brightness in projection systems emphasizes low f/# optics. However, as is noted above with reference to FIGS. 2a-2d, the performance of dichroic surfaces used for combining color image paths is hindered by the large incidence angles of low f/# optical systems.
In spite of their high cost and known problems, X-cubes have served as dichroic combiners in a substantial number of imaging device designs. One reason for the widespread use of X-cubes and similar devices relates to their compact size when compared against other solutions using dichroic surfaces. In order to minimize back working distance of the projection lens, conventional design approaches position spatial light modulators closely to the X-cube, as was shown in U.S. Pat. No. 6,113,239, for example.
Strategies for eliminating the X-cube include those disclosed in U.S. Pat. No. 6,321,192 (Konno et al.) Here, dichroic mirrors are employed for combining the light from each color modulation path, with equal optical distances maintained in each light path. The system described provides a less costly solution than conventional solutions using X-cube optics. At the same time, however, the solution disclosed in U.S. Pat. No. 6,231,192 does not alleviate the back working requirements of the projection lens system. This solution limits the f/# of projector optics to slower speeds, constraining the available brightness and requiring larger projection lens diameters. Correction for astigmatism must be provided by cylindrical optics in the projection path.
Dichroic components, including X-cubes, Philips prisms, and related structures that use dichroic surfaces, are also used in the illumination light path, conventionally employed to separate white light into red, green, and blue light components for modulation. U.S. Pat. No. 6,053,615 (Peterson et al.), U.S. Pat. No. 6,220,713 (Tadic-Galeb et al.), and U.S. Pat. No. 6,254,237 (Booth) are just a few examples of projector designs using dichroic components for color separation. As can be expected, the same performance problems related to light incident angle apply whether dichroic coatings are used to combine modulated light into the projection path or are used to separate unmodulated light in the illumination path. When dichroic surfaces receive incident light at varying angles, output performance is affected, causing perceptible color shifts across the field. It is difficult to correct for this slight color shading; graduated filters are expensive to design and reduce the overall brightness available. With respect to color accuracy, optimum results are obtained when dichroic surfaces handle incident light at relatively low incident angles. This provides the best filter response characteristics, allowing minimal leakage of unwanted color in each color channel. Referring to FIG. 4, there is shown a typical reflectance response, by wavelength, for a dichroic surface. The set of curves show the successive variation in response for the same dichroic surface as the incident angle changes over a range. At the extremes of this range of incident angles, curves 150a and 150z show how dramatically this response can be altered. Here, the response curve changes its reflectance characteristics with incident angle, so that with the incident angle at one extreme the primary transition occurs near 520 nm, as shown by curve 150a, and at the other extreme the primary transition occurs just above 620 nm, as shown by curve 150z. Since incident light will vary over some range, there can easily be some leakage of green light into the red channel, for example. Thus, it can be appreciated that limiting the range of incident angles handled by a dichroic surface will provide the best color performance. It is also instructive to compare the steeper slope of curve 150a with the less steep slope of curve 150z. As a general rule, the steeper the slope of the response curve for a dichroic surface, the better the spectral characteristics, that is, the less leakage of unwanted wavelengths. Sharply defined spectral edges are desirable for obtaining the best color response from a dichroic surface.
One solution for maximizing brightness and reducing the overall incident angles at color combiner dichroic surfaces is to increase the size of the uniform light area and effectively lower the numerical aperture of the uniformized light area that is incident on dichroic separator surfaces. However, this would be difficult to achieve without increasing the bulk and cost of uniformizing components. Further problems present themselves. For example, where an integrating bar is used as a light uniformizer, it would not be sufficient merely to make the integrating bar larger in cross-section. In order for sufficient homogenizing of the light, the integrating bar would also need to be longer, adding size, weight, and cost. Similar problems limit solutions using other types of uniformizing components. Such solutions are made more difficult by the need to maximize light at the spatial light modulator itself, providing incident light at a high numerical aperture.
In summary, conventional approaches to digital projector design make some compromises relative to performance, cost, and complexity in order to provide a high quality image with maximum brightness. Existing designs admit some degree of color shift; conventional approaches for correcting color shift would require more costly dichroic coatings and entail some additional loss of brightness in both illumination and modulation optics paths. Thus, it can be seen that there is a need for improvement in illumination and modulation path optics for digital projection that alleviates the inherent angular limitations of lower cost dichroic coatings while providing maximum brightness and color gamut.